The object of this invention is a measuring device for measuring small forces and displacements having in a body of the device a detector means comprising a light source and a position-sensitive detector, and a sensing means comprising a spring means to be loaded by a force to be measured and a shading means mounted to this spring means, which shading means moves under the influence of the force loading the spring means in a light beam of the light source of the detector means, the beam being directed towards an active surface of the detector.
The patent publication FI 963612 discloses a device for measuring a surface pressure of a film formed at the interface of a liquid and gaseous phase. In the device known from this publication, a position-sensitive detector is employed together with a light source in the detection of the displacement of a sensor caused by the small force to be measured.
In this known device the compensation of errors due to the temperature becomes, however, difficult and tedious in a wide range of temperature. Although a part of the errors can be corrected with calibration procedures and/or by using suitable compensation solutions e.g. in the control electronics of the devices, these calibration and compensation procedures become tedious and laborious. Other known devices, like ordinary microbalances, are on their part expensive, spacetaking and they cannot be thought as portable measuring devices.
The aim of the invention is to provide a device suitable for measuring small forces, like e.g. surface tension and surface pressures and small displacements, where the influence of the temperature of the environment to the measuring result is very small. In addition, the object is to provide a measuring device which has small physical dimensions, which is portable and suitable also for field work outside the laboratories.
This is accomplished according to the invention so that the detector means and the sensing means are arranged into a common clamp arranged in the body of the device.
According to a preferable embodiment of the invention, the detector means and the sensing means are arranged into a common clamp made of a material having a small coefficient of temperature expansion, preferably e.g. of bakelite. Other preferred alternatives are e.g. ceramic materials. In this embodiment the relative mutual position of the detector means and the sensing means remain the same while the temperature of the environment is varying. The clamp has preferably an U-formed shape, the light source and the position-sensitive detector of the sensing means being located towards each other in different legs of the said U-formed clamp, and the shading means of the sensing means being at least partially in the space between these legs.
According to the invention the detector means comprises a light source, which is preferably e.g. a light emitting diode, and a position-sensitive detector.
The position-sensitive detector is as such of a known type which comprises a silicon substrate and a layer structure formed thereon. The outermost layer is resistive, that is an active P-layer. The incident light which falls on the active P-layer is converted to an electrical charge which is proportional to the light energy. This charge is driven through the P-layer to electrodes connected to the layer. As the resistivity of the layer is constant, a photo current is obtained at the electrodes, which is inversely proportional to the distance between the incident light spot and the electrodes.
Between the light source and the position-sensitive detector, preferably close to the light source, optical means, preferably a lens, is arranged in a preferred embodiment of the invention for directing the light beam of the light source towards the active area of the light-sensitive detector. The light emitted from the light source can be in the visible or in the infrared spectrum of light. In a preferred embodiment of the invention, the intensity of the light emitted by the light source is modulated.
According to the invention, the spring means of the sensing means transforming the force applied thereto into a linear movement, is preferably a leaf spring, made of a material having a low coefficient of thermal expansion, preferably of e.g. quartz glass. Other preferred alternatives are e.g. ceramic materials. In a preferred embodiment of the invention this spring means of the sensing means has a substantially lying U-shaped form so that the imaginary plane through its legs is vertical and that the end of its one leg is mounted to the said clamp and to the end of the other leg a preferably planar shading means is arranged, the shading means having means for bringing the force to be measured into connection with the spring means.
The shading means arranged to the spring means is arranged according to a preferred embodiment of the invention between the light source and the detector substantially perpendicularly to the light beam of the light source of the detector means, the beam being directed towards the light-sensitive detector. The shading means has in a preferred embodiment of the invention such a form that its edge projecting to the light beam is straight and preferably perpendicular to the direction of the applied force at least in that part which is within the light beam. When the shading means moves in the light beam under the influence of the force to be measured applied to the spring means, a part of the light emitted by the light source hits the active surface of the light-sensitive detector of the sensing means.
In another preferred embodiment of the invention, a slot or an opening is arranged to the planar shading means through which a part of the light beam of the light source directed towards the active surface of the detector hits the detector. The slot is preferably arranged so that it is across the direction of the movement of the shading means when this moves in the said light beam under the influence of the force to be measured. In a preferred embodiment of the invention, optical means is arranged between the shading means having a slot and the detector in such a way that these optical means form an image of the slot onto the active surface of the detector. In a preferred embodiment these optical means consist of a mirror. The detector is placed in this embodiment preferably e.g. to the horizontal part of the U-shaped clamp or even to the same leg with the light source.
In a preferred embodiment of the invention, the clamp into which the detector means and the sensing means are arranged is mounted to the body of the device with suitable suspension means for bringing this clamp in such a position where the force deflecting the spring means of the sensing means is substantially perpendicular to the spring means and in a vertical plane going through the spring means. In this preferred embodiment the lateral movement of the spring means can be avoided.
In another preferred embodiment of the invention, the device comprises means in communication with the suspension means for sensing the position of the clamp.
The device according to the invention can preferably be used for weighing small masses, preferably in a measuring range of 0-200 mg, and a resolution of 0.1 xcexcg can be attained in this measuring range. The attainable resolution depends on the resolution of the position-sensitive detector employed.